The present invention relates to a glazing system for a building, to mountings and fittings for use in such glazing systems to secure a glazing panel to a building, and to methods of improving the impact resistance of a glazing panel.
Frameless glazing systems for buildings are well known. Buildings incorporating a structural glass facade or curtain wall incorporating the Pilkington PLANAR™ glazing system are one such example. This type of structural glazing system comprises a plurality of glazing panels ranging typically in size between 1 m×1 m and 2.5 m×4.5 m, wherein each glazing panel is independently secured to an adjacent support structure via mechanical mountings and fittings. Each glazing panel usually comprises a sheet of glass that has been toughened and may be laminated to meet specific legislative requirements. Each glazing panel may be double or triple glazed to provide increased environmental performance.
In such frameless glazing systems, each glazing panel usually has a number of holes drilled through it, which may be straight or countersunk bores, for securing a mechanical fitting thereto. For a rectangular glazing panel the fixing holes or bores are usually in the vicinity of each corner and a mechanical fixing is connected to the glazing panel via each bore. Depending upon the size of the glazing panel, additional holes may be located at other positions around the periphery of the panel.
A glazing fitting passes through a bore in the glazing panel and is secured to the glazing panel by a suitable screw assembly. The glazing fitting usually has a hook that projects into the interior of the building. The hook is then connected to a suitable mounting comprising a mounting member that is secured to a suitable adjacent structural support element forming part of the building. The glazing fitting and mounting member are usually made of the same grade of stainless steel although mechanical strengths can be varied as required by design. An assembly comprising such a glazing fitting and mounting are used with the Pilkington PLANAR™ structural glass systems, commercial examples of such assemblies being known as 905 fittings.
When used as an exterior wall or facade on a building, such glazing fitting and mounting assemblies are specified by the engineer to be of sufficient strength to support the applied wind load. Equally, the two elements of the assembly are designed with sufficient tolerance and clearances to provide for adequate thermal expansion and building movement by rotation and translation of the mounting member within the glazing fitting which is attached to the glazing panel.
However for certain weather conditions the pure mechanical strength of an assembly designed in a conventional manner for wind load, thermal expansion and building movement is not sufficient. For example, in hurricane conditions, where airborne debris can impact upon the glazing panels, the standard mechanical assembly may not have sufficient strength or flexibility to withstand such a mechanical impact.
The impact resistance of a glazing system is influenced by many factors, for example, the material from which the glazing panel is made (which is usually glass), whether the glazing panel is laminated or not, the type of interlayer material specified for the laminate and the size of the glazing panel.
For a glazing panel comprising a single glass sheet, toughening the glass sheet (either thermally or chemically) strengthens the glass sheet thereby improving impact resistance. Thermally toughened glass sheets may be heat soaked to avoid potential problems with nickel sulphide inclusions. A glazing panel comprising two glass components and an interlayer material such as PVB is another known method of improving impact resistance. Dependent upon the particular application, for a glazing panel to be classified as impact resistant, the panel will usually be required to meet certain legislative standards by passing recognised test methods appropriate to the application.
To be classed as hurricane impact resistant in certain states of the USA, the glazing system must comply with specific Building Codes. In particular, the International Building Code (IBC), the International residential Code (IRC), the Florida Building Code (FBC) and the Texas Department of Insurance (TDI) form a central part of the minimum building standards designed to protect buildings from high wind events such as hurricanes. This family of building codes considers, amongst other things, curtain walls as part of the building envelope which may therefore be subjected to damage from flying debris during a high wind event. Prevention of a building envelope breach is critical not only to the continued structural integrity of the building due to resultant over pressurisation, but also to the protection of the building contents and occupants. Internationally recognised standard ASTM E 1996-08 provides a standard specification for defining the performance of a glazing system when impacted by windborne debris in a hurricane.
At present hurricane impact performance of windows has been improved by using laminated glazing panels. A typical hurricane impact resistant laminated glass panel comprises an interlayer material bonded between two panes of glass. A particularly suitable interlayer material is available from Du Pont™ and is sold as SentryGlas®. Compared to a conventional PVB interlayer, SentryGlas® is more rigid and tough and is therefore able to contribute to the increased ability of a glass panel to remain unbreached and in place when subjected to the above mentioned impact tests.
For a glazing panel comprising a glass sheet that is supported by mechanical fittings via holes in the glazing panel rather than being fitted into a conventional frame, the limiting factor for the glazing panel wind load capacity and impact resistance becomes the area of glass and interlayer which is in the immediate vicinity of the holes through which the glazing fittings pass. In this area surrounding the hole, the interlayer and glass are both vulnerable to higher localised loads, distortions and stresses. This makes achieving the required level of wind load and impact performance for such a glazing assembly difficult.
This type of generic bolted system has previously been known to secure panes of glass in the construction of frameless display cases, shop windows, feature glazing for commercial buildings and the like. One example of such a generic bolted system is described in GB 311,616. The shank of a screw passes through a hole in a glass sheet and is screwed into a clip. A rubber washer backed by a metal washer is placed under the screw head and the glass pane is secured by the clip and the washers. Such an attachment assembly is not suitable for providing a glazing assembly that will pass current impact legislation, in particular the hurricane high wind load and impact tests mentioned above.
In JP08-333,831A a fastener for a wall body panel is disclosed. The fastener is used to improve the wind resistance of the panel. For a wind load applied substantially normal to the body panel, the fastener allows the body panel to move in the direction of the applied wind load.
FR 2 738 271 discloses a fixing for attaching a glass panel to a building. The fixing has a support shaft having a bulged portion along the central portion thereof. A glazing fitting having a hook portion is attached to a glass panel and the fixing is connected to the hook portion via the bulge portion. Such a fixing finds particular application against the effects of wind load.
It is possible to improve the impact performance of a laminated glazing panel by using an interlayer that extends beyond the panes of glass in the laminate, as described in US2003/0188498A1. The exposed interlayer is then bonded into the surrounding framework or onto adjacent metal lugs forming part of the framework. This solution is not possible for a frameless glazing assembly of the type described above, because there is no frame surrounding each glazing plane into which the interlayer can be bonded.
Another solution described in the October 2007 edition of Glass Magazine is to bond a glazing fitting onto a face of the glazing pane using a strong structural adhesive, thereby eliminating the need to have any holes in the glazing panel at all and the associated vulnerabilities. Such systems have been developed for frameless glazing systems such as curtain walls and facades, but require the use of specialist adhesives that are applied in controlled environments to achieve the necessary durability and UV stability of the connection. There can be high costs associated with such a method.